A disc brake pad for a disc brake of a motor vehicle generally is a combination of a steel back plate and a friction material fixed on the steel back plate. The friction material may be formed by pressurizing and heating a mixture of powdered materials including such as a fibrous material, a filler, and a binder. A disc brake brakes a motor vehicle with a frictional force generated as pressing the disc brake pad against a metal disc rotor.
However, conventional disc brakes have problems of making brake noises because of disc brake pad and/or disc rotor vibration during the brake operation. Such brake noises have been developed to minimize the disturbing brake noises. One conventional method to prevent the brake noises is to form inclined surfaces at both ends of the disc brake pad.
FIGS. 5 and 6 represents resultant disc brake pads in general. Appearances of the resultant disc brake pads are the same for both the present invention and the conventional arts; however, the differences between the present invention and the conventional arts reside in the process of manufacturing the disc brake pad, contents, and the effect of the resultant products. FIG. 5 illustrates the resultant disc brake pad in general with the inclined surfaces, where FIG. 5(a) is a plan view thereof and FIG. 5(b) is a front view thereof. A disc brake pad 1 as shown in FIG. 5 is a combination of a steel back plate 2 and a friction material 3 fixed on the back plate 2. The friction material 3 has a plane friction surface 3a on an intermediate portion thereof and inclined surfaces 3b, 3b at both side portions thereof. Borderlines 5, 5 between the friction surface 3a and the inclined surfaces 3b, 3b are parallel. Accordingly, the length of the friction surface 3a contacting an outer circumferential side of the disc rotor is designed to be substantially equal to the length of the frictional surface 3a contacting an inner circumferential side of the disc rotor.
Forming such inclined surface 3b on the disc brake pad 1 prevents the disc brake pad 1 from scratching the disc rotor, thereby eliminating/minimizing the brake noises.
FIG. 6 illustrates another resultant disc brake pad in general with inclined surfaces. The inclined surfaces 3d, 3d of FIG. 6(a) are formed so that an upper portion of the friction surface 3c expands toward outside of the disc rotor to form a fan shape.
Vibration of the disc brake pad 1 and/or the disc rotor during the brake operation are considered to be causes of the brake noises, and the borderlines on the disc brake pad becomes a right angle against the tangent line of the disc rotor, thereby providing more effective vibration control on the disc rotor. Accordingly, the conventional friction surfaces are formed in the fan shape as shown in FIG. 6(a).
FIG. 7 is a flow chart explaining a general process of manufacturing a disc brake pad using a metal forming die. In FIG. 7, mixing and step for the powdered raw material and a preliminarily forming step for the mixed powdered raw material take place while washing and surface-treated steps for the steel back plate take place. Further in this process, the preliminarily formed product is fixed on the steel back plate to go through forming, curing, and polishing steps. If the above-explained inclined surfaces 3b, 3b, 3d, 3d, 3f, 3f, are formed by grinding the friction material surface, such grinding step should take place after a polishing step in FIG. 7 is typically needed. Alternatively, the grinding step can be eliminated to simplify the entire process and to minimize grinding dusts by utilizing the metal forming die, where the step of forming the inclined surfaces takes place in a preliminary forming and forming steps in FIG. 7.
Patent Document 1 illustrates a conventional manufacturing process for a preliminarily formed product with a temporary inclined surface by pressurizing and molding the powdered raw materials without heating in a metal preliminarily forming die and a manufacturing process of the friction material with a final inclined surface in a desirable angle by heating and pressurizing the preliminarily formed product in a metal forming die with an opening same shape as the opening of the metal preliminarily forming die, so as to provide a larger inclined surface angle of the preliminarily formed product than the inclined surface angle of the friction material.
In the Patent Documents 1, the temporary inclined surface formed on the preliminarily formed product has a steeper than the final inclined surface to be formed on the friction material. The inclined surface of the metal forming die is designed to contact only inside of the temporary inclined surface of the preliminarily formed product leaving a wage shape space at the outside of the contacting portion, and therefore the powdered raw materials is melted by heating; however, because of the space reserved between the metal die and the preliminarily formed product, the raw material flows to provided a entirely uniform friction material.
Yet, when the amount of the fibrous materials in the friction material is large and the flowability of the powdered raw materials is not sufficient, the manufacturing method of the Patent Document 1 cannot often provide the entirely uniform friction material.
More specifically, there is not enough space for the powdered raw materials to flow and not enough flowability of the powdered raw materials, thereby providing a disc brake pad having a friction material with a higher density at the final inclined surface portion thereof. As such that the high density of the friction material at the final inclined surface portion, when the final inclined surface portion disappears due to the friction material wear, brake noises tend to occur upon the disc rotor frictionally engaging the high density friction material without the final inclined surface.    [Patent Document 1] Japanese Provisional Patent Publication No. 2006-83978